9701 · 13.2
Characteristic organic reactions — FAQ
Frequently asked questions for 9701 Characteristic organic reactions. Direct answers first, then deeper explanation — then practise with marking.
Why are alkanes so unreactive compared to alkenes?
Alkanes contain only sigma (σ) bonds, which are strong and have low polarity. There are no lone pairs or regions of high electron density to attract attacking species (electrophiles or nucleophiles). Alkenes, however, have a π-bond, which is a region of high electron density and is weaker than a σ-bond, making it susceptible to attack by electrophiles.
What is the difference between homolytic and heterolytic fission?
Homolytic fission is the symmetrical breaking of a covalent bond where each atom gets one electron from the shared pair, forming two free radicals. This is common in non-polar bonds initiated by UV light. Heterolytic fission is the unsymmetrical breaking of a bond where one atom takes both electrons, forming a positive ion (cation) and a negative ion (anion). This is common in polar bonds.
How do I know if a nucleophilic substitution reaction is Sₙ1 or Sₙ2?
The primary factor is the structure of the halogenoalkane. Tertiary (3°) halogenoalkanes favour the Sₙ1 mechanism because they can form a relatively stable tertiary carbocation intermediate. Primary (1°) halogenoalkanes favour the Sₙ2 mechanism because there is little steric hindrance, allowing the nucleophile to attack the carbon atom directly. Secondary (2°) halogenoalkanes are intermediate and can undergo both mechanisms.